Hillary Clinton's chances of winning the Democratic nomination seem to be slipping away as the long march to the Pennsylvania primary empties her war chest as fast as she can fill it.

At the same time, more and more super-delegates – the Democratic Party bosses who hold the balance of power in the nomination process – have backed Barack Obama or indicated that they intend to do so in the weeks ahead.

Prominent figures such as former president Jimmy Carter have all but declared for Mr Obama.

"My children and their spouses are pro-Obama. My grandchildren are also pro-Obama," Mr Carter told a Nigerian newspaper while in Africa. "As a super-delegate, I would not disclose who I am rooting for, but I leave you to make that guess." Other super-delegates who officially back Mrs Clinton are looking to the future and preparing to jump ship if she does not trounce her opponent in the Pennsylvania primary in three weeks time.

That increasingly competitive and costly race is followed by elections in North Carolina and Indiana two weeks later.

Mr Obama's fundraising continues to outpace that of Mrs Clinton, and he is now outspending her in political advertising by a rate of more than two to one in Pennsylvania, and doing better in the polls as a result.

The latest New York Times national poll indicates that Mr Obama has maintained a narrow lead of 46 per cent to 43 per cent over Mrs Clinton. But the survey also shows that his support among Democrats has grown weaker over the past month as the crisis over his controversial former pastor reached fever pitch. Men and upper-income voters have taken an increasingly dim view of Mr Obama since his surge of victories in February.

The poll found that Mr Obama's popularity rating since those successes has dropped by seven points, to 62 per cent.

But the fall is nowhere near the collapse that the Clinton campaign was hoping for as it seeks to make the case that Mr Obama is unelectable.

The poll reveals that 56 per cent of Democratic primary voters say he can beat the Republican John McCain in November, compared to just 32 per cent who think Mrs Clinton is the more electable.